Central Maine Orthopaedics, a 12-physician practice in Auburn, and OA Centers for Orthopaedics, a 16-physician practice in Portland, announced plans late Thursday to explore a merger with the much larger Spectrum Medical Group. Based in South Portland, Spectrum employs more than 170 physicians in Maine and New Hampshire in a range of medical specialties.

The three organizations have signed a letter of intent to explore a possible merger, according to a release from the three groups. The Auburn and Portland practices jointly approached Spectrum after deciding that consolidating would best serve their patients, the release states.

Spectrum, which consists of nine divisions, is the largest independent medical practice in northern New England.

“Both OA and CMO have a strong focus on delivering high-quality orthopaedic care that will improve patients’ quality of life,” Michael Cox, chief executive officer of CMO, said in the release. “Under the Spectrum umbrella, we would be able to achieve our mission more effectively.”

Spectrum’s specialties include anesthesiology, critical care medicine, radiology, neurology, pathology, physical medicine and rehabilitation, radiation oncology and surgery. Integrating Central Maine Orthopaedics and OA Centers for Orthopaedics would create an orthopaedics division, bringing the total number of physicians to nearly 200.

The three practices expect to complete a merger by the end of 2013. The organizations plan to work with the state’s certificate of need unit and other government agencies to determine the scope of regulatory review that the proposed merger will require.

“It’s certainly the largest consolidation of independent, privately owned medical practices in the state in recent times,” said Gordon Smith, executive vice president of the Maine Medical Association.

Spectrum has been active over the last year in bringing additional specialties into its fold, Smith said. Spectrum recently integrated two Bangor practices, Neurology Associates of Eastern Maine and Penobscot Surgical Care.

In announcing the merger plans, the three organizations underscored that remaining independent — while many other physician practices have been scooped up by hospitals — would allow them to deliver services more efficiently and at a lower cost.

“Independent practices offer physicians a collaborative work environment and an increased ability to directly influence initiatives that affect patient care,” Douglas Brown, president of OA, said in the release. “In a health system-dominated market, independent practices preserve choice for both patients and physicians.”

Many moderately sized physician practices have responded to the pressures of a changing health care landscape — including payment reforms and the adoption of electronic medical records — by joining forces, Smith said.

“These are practices that are very committed to continuing to practice independently owned,” he said.

The merger would not affect patients’ relationships with their individual physicians or their day-to-day interactions with the practices, according to the release. All of the practices’ locations would be maintained, including OA Centers for Orthopaedics’ satellite facilities in Windham, Saco and Brunswick. No staffing changes are planned.

Spectrum and OA Centers for Orthopaedics have collaborated previously. In 2011, the organizations joined with two other independently owned physician practices to form Specialty Solutions, an association of 10 independent physician practices.

“We are thrilled about this opportunity with OA and CMO,” Daniel Landry, president of Spectrum, said in the release. “They are both vital and financially sound practices that share a consistent vision with Spectrum. Our collaboration would allow us all to more efficiently achieve our collective goal of providing high-quality, cost-effective, patient-centered care.”